Studies based on nuclear magnetic resonance (NMR) entail the application of a magnetic field B1 oscillating in the radio-frequency (RF) range (i.e., approximately 4–900 MHz) to a sample while the sample is subjected to an external, static magnetic field B0 that is much stronger (typically two or more orders of magnitude) than the B1 field. If a resonance condition is fulfilled, the relatively small periodic perturbation caused by the B1 field can produce a large change in the orientation of the net magnetization of certain types of nuclei in the sample via transfer of electromagnetic energy. This phenomenon occurs because NMR-active nuclei, i.e., the types of nuclei responsive to NMR, possess the fundamental property of spin and can behave as magnetic dipoles. A spinning, charged nucleus creates a magnetic moment oriented along the axis of the spin. Commonly studied NMR-active nuclei include, but are not limited to, 1H (a proton) and the isotopes 13C (carbon-13), 19F (fluorine-19), and 31P (phosphorous-31). At a given instant of time, such nuclei are observed to exist in one of two magnetic quantum states, m=+½ (the lower energy state) and m=−½ (the higher energy state) and thus are termed spin-half nuclei (other nuclei with non-zero nuclear spins, but where m≠±½, can also be studied; typical examples include 2H, 23Na, and 14N). If a nucleus of this type is subjected to a B0 field, its magnetic moment becomes oriented in one of two directions relative to the direction of the B0 field depending on its magnetic quantum state. For a positive γ, at the lower energy state corresponding to m=+½ the magnetic moment is aligned with the B0 field, whereas at the higher energy state corresponding to m=−½ the magnetic moment opposes the B0 field. The application of a B1 field causes a transition between energy states if the frequency of the applied RF energy is at or near the Larmor (or resonance) frequency v0 according to the expression v0=(γB0)/2π. The B1 field can be induced by current alternating at or near the resonance frequency and running through a transmitter coil that surrounds the sample. The resonance frequency thus depends on the strength of the polarizing magnetic field B0 and the intrinsic properties of the nucleus as reflected in the proportionality constant γ, known as the gyromagnetic ratio of the nucleus. Different types of nuclei have different gyromagnetic ratios and thus, for a given B0 field, require a different resonance frequency for excitation. A transition from a lower energy state to a higher energy state corresponds to absorption by a nucleus of electromagnetic energy, i.e., a photon whose energy E=hv=hγB0/2π (where h is Plank's constant) matches the energy difference between the two states. A transition from a higher energy state to a lower energy states state corresponds to emission of electromagnetic energy by the nucleus.
In the presence of the applied B0 field, NMR-active nuclei in the sample become oriented such that the lower energy state predominates and thus a net adsorption of electromagnetic energy exists. For a positive γ, the net magnetic field attributed to the magnetic moments of the nuclei, i.e., the bulk magnetization of the sample, points in the direction of the applied B0 field. The relative excess of nuclei in the lower energy state is on the order of ppm (parts per million) and can be determined from Boltzmann statistics. The difference between the energy absorbed by nuclei transitioning from the lower energy state to the higher energy state, and the energy emitted by nuclei simultaneously transitioning from the higher energy state to the lower energy state, is manifested in an NMR response signal. That is, the NMR response signal is proportional to the population difference between the two energy states. In effect, the B1 field is applied in such a way as to cause the magnetization vector to tilt away from the axis along which the B0 field is directed (usually taken to be the z-axis) and to precess about the axis. This precession induces a current in a receiver coil surrounding the sample; in pulse NMR techniques, the same coil can be used as both the receiver coil and the transmitter coil. The current, or NMR response signal, detected by the coil can be amplified and processed to yield an NMR spectrum in the frequency domain. The spectrum consists of one or more peaks whose intensities represent the proportions of each frequency component present. The excess number of nuclei in the lower energy state increases with decreasing temperature and increases as a function of Boltzmann's constant (e−ΔE/KT) with increasing B0 field strength. Hence, at a given temperature, the intensity of the NMR response signal derived from the excess increases linearly with increasing B0 field strength. The frequency at which a nucleus can absorb RF energy is influenced by its local chemical environment (e.g., nearby electrons and nuclei). Commonly studied environmental effects include chemical shifts arising from ancillary magnetic fields created by circulation of electrons around nuclei, and spin-spin splitting arising from couplings between nearby nuclei. Thus, NMR spectra can provide useful information indicative of molecular structure, position, and quantity in chemical, biochemical, and biological species of interest.
Most modern NMR systems utilize a Fourier transform (FT) NMR spectrometer to provide the B1 field and a magnet to provide the B0 field. Although electromagnets and permanent magnets have been employed in the past, a superconducting magnet is preferred because it can produce a more powerful (typically on the order of several Tesla), homogeneous, and reproducible B0 field. The superconducting magnet typically includes a solenoidal wire wound around a magnet bore and a cooling system such as a cryostat employing liquid-helium and liquid-nitrogen heat sinks to maintain the wire at the critical temperature required for superconductivity. If necessary, the system can include components for compensating for drift and inhomogeneity in the B0 field generated by the magnet. Such components may include a field/frequency lock system that irradiates a reference nucleus such as deuterium to produce a reference signal for drift correction, and shim coils for offsetting spatial inhomogeneities in the B0 field. A sample containing NMR-active nuclei to be investigated is held in a sample container and mounted in a sample probe within the magnet bore where the sample can be immersed in the B0 field. RF electronics of the spectrometer supply RF energy to the transmitter coil located in the sample probe. The RF energy is supplied in the form of a pulse or a precisely controlled sequence of pulses that are prescribed according to the type of sample and the chosen parameters of the experiment, thereby inducing the transmitter coil to irradiate the sample with the appropriate B1 field. Typically, the direction of the applied RF pulses is orthogonal to that of the main magnetic field B0. If the resonance conditions are fulfilled for a given type of nucleus, the RF pulses excite the nucleus to a degree sufficient for detection. During the delay interval between pulses, the nucleus emits an RF time-domain signal, known as a free induction decay (FID) signal, which decays in the interval as the excited nucleus relaxes back to an equilibrium state. A separate coil can be used to detect the FID signal, but since the excitation pulse and FID occur during different time intervals, a single coil contained in the sample probe can function as both the transmitter and receiver as previously noted. Fourier transformation then converts the time-domain signal to a frequency-domain signal to produce an NMR spectrum interpretable by the researcher.
It can be appreciated that because the sample coil serves as the direct interface for the coupling of pulsed RF excitation and detection signals between the sample and the NMR system, the design and performance of the sample coil are critical. This is especially true in the case of a multi-tunable (or multi-resonant) coil that is capable of being tuned so as to transmit two or more resonant frequencies for independently exciting two or more different nuclei—i.e., multi-pulse, multinuclear, and multi-dimensional techniques in which nuclei of differing types (e.g., 1H and 13C) and/or chemically non-equivalent nuclei of the same type are excited for such purposes as observation, decoupling, polarization transfer, etc.
Coils of conventional design include solenoid coils, saddle coils, loop-gap resonators, Hemholtz, birdcage, slotted tube (e.g., Alderman-Grant-style resonator), and combinations of two or more types of these coils. Each type of single-coil and multi-coil design has advantages and disadvantages and thus trade-offs among various performance-related factors have been unavoidable for the most part. Some of the merits and demerits of certain coil designs are summarized by Koskinen et al., “The Concentric Loop-Gap Resonator—A Compact, Broadly Tunable Design for NMR Applications,” J. Magnetic Resonance, 98, 576–588 (1992).
A solenoid coil is prone to transferring an undesirably large amount of heat energy to the sample mounted in its core, which in practice limits the range of experiments available to the researcher, especially experiments performed at high frequency. Sample heating can be attributed, at least partially, to the fact that the solenoidal structure impresses a potential difference along its longitudinal axial length and hence also along the longitudinal axial length of the sample. Consequently, an energized solenoid coil creates non-conservative electrical fields in its core of significant strength. A sample inserted in the core is immersed in this electrical field. Sample heating can destroy or at least degrade a thermally labile sample. In addition, solenoid coils have required so called “balun” technology (see, e.g., U.S. Pat. No. 6,380,742, assigned to the assignee of the present disclosure) to optimize matching of B1 field distributions for different nuclei, but nonetheless experience a shortened B1 field for high-frequency operation (typically 1H) due to wavelength effects. In addition, all conventionally designed sample coils of a given size exhibit an RF field whose homogeneous region is less than optimal. This often means that the size of a sample to be irradiated by the sample coil is limited. For a given coil size, the result is lower sensitivity than would be realized if a greater amount of the sample could be exposed to the homogeneous portion of the field. In addition, many conventional sample coils have been designed to exhibit low inherent inductance to minimize electrical field coupling. Examples are loop-gap resonators and Alderman-Grant-style resonators. These coils are fundamentally limited to a single turn, resulting in a fixed and typically very low inductance. Unfortunately, this low inductance results in insufficient performance for low-frequency nuclei.
Multi-coil (typically dual-coil and triple-coil) designs have been developed to address many of the problems attending conventional single-coil designs, but present additional problems. One obvious problem relates to the fact that the sample probe must accommodate more than one sample coil and hence engenders mechanical and electrical challenges. For example, mechanical difficulties inhere in the design of sample probes that must be capable of spinning the sample, particularly when the angular velocity and/or the spinning angle must be precise, as well as the generation of artifacts contributed by vibrations and reliability issues. Electrical difficulties include the challenge of ameliorating unwanted couplings between the various coils or other parts of the RF circuitry.
It would therefore be desirable to provide a sample coil that combines advantages of single-coil and multi-coil designs while eliminating or at least reducing their disadvantages. For instance, it would be desirable to provide a sample coil that exhibits the relative technical simplicity of a single-coil designs as well as the substantially reduced electrical field coupling commonly only obtainable with relatively complex dual-coil designs. Moreover, it would be advantageous to provide a sample coil that produces a substantially larger homogeneous magnetic field for a given physical coil size and a better match of the B1-field distributions for different nuclei even at very high frequencies, whereas conventional solenoidal designs, for example, are limited by wavelength effects and two-coil designs experience limitations due to the fundamental difference between the respective sizes and geometries of the two coils. In addition, it would be advantageous to provide a sample coil that inherently causes less sample heating, thus broadening the range of the types of samples that can be analyzed.
To address the foregoing problems, in whole or in part, and/or other problems that may have been observed by persons skilled in the art, the present disclosure provides sample coils exhibiting a scroll geometry, as described by way of exemplary implementations set forth below.
The utility of the scroll geometry for RF applications has, in limited extent, been preliminarily investigated. See Gimi et al., “Investigation of NMR Signal-to-Noise Ratio for RF Scroll Microcoils,” 1st Annual International IEEE-EMBS Special Topic Conference on Microtechnologies in Medicine & Biology (Oct. 12–14, 2000). These authors performed an NMR spectroscopy experiment on de-ionized water, utilizing a manually wrapped scroll coil tuned and matched to resonate at 500 MHz, an NMR magnet having a field strength of 11.75 T, and a 90-degree pulse. From these testing parameters, it appears that the authors performed a basic, single-resonance, proton NMR spectroscopy experiment. Moreover, the authors reported the occurrence of line-broadening in their resulting spectra, leading to their conclusion that scroll coils in their current state are not desirable for spectroscopy, as opposed to imaging for which considerations of spectral resolution are less restrictive. Wiltshire et al., “Microstructured Magnetic Materials for Radio Frequency Operation in Magnetic Resonance Imaging (MRI),” article submitted to Science (Dec. 10, 2000), reported using a bundle of “Swiss Rolls” as a flux-guiding medium in an MRI experiment. A solenoid detector coil tuned to 21.3 MHz was placed immediately below a water phantom, the bundle of “Swiss Rolls” was placed on the water phantom, and the object to be imaged was placed on the bundle. The bundle of “Swiss Rolls” was thus employed to couple the object to the solenoid detector coil. No suggestion was made that the “Swiss Roll” structure could itself be employed as a transmitting or receiving coil.
Therefore, an ongoing need exists for improved coil designs for transmitting and/or receiving RF signals in magnetic resonance-related processes, and particularly designs that overcome one or more of the disadvantages mentioned above or other disadvantages encountered in the art.